Auxins

Auxins (from the Greek word 'auxein', meaning 'to grow') were first isolated from human urine. The term auxin is applied to indole-3-acetic acid (IAA) and to other natural and synthetic compounds that have certain growth-regulating properties. Auxins are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.

• Auxins such as IAA and indole butyric acid (IBA) have been isolated from plants.
• NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are synthetic auxins.
• All these auxins have been used extensively in agricultural and horticultural practices.

Some important effects and uses of auxins are:

• They help to initiate rooting in stem cuttings, an application widely used in plant propagation.
• They promote flowering, for example in pineapples.
• They help to prevent fruit and leaf drop at early stages, but promote the abscission of older, mature leaves and fruits.
• In most higher plants the growing apical bud inhibits the growth of the lateral (axillary) buds — a phenomenon called apical dominance. Removal of the shoot tips (decapitation) usually results in the growth of the lateral buds; this is widely applied in tea plantations and hedge-making.
• Auxins induce parthenocarpy (the development of seedless fruit), for example in tomatoes. …